The present invention relates generally to nitride based semiconductor structures, and more particularly to a nitride based resonant cavity semiconductor structure with highly reflective mirrors on both sides of the semiconductor structure. The highly reflective mirrors are typically distributed Bragg reflectors (DBR), but can also be simpler metal terminated layer stacks of dielectric materials.
A planar multi-layered semiconductor structure can have one or more active semiconductor layers bounded at opposite sides with layers that form distributed Bragg reflectors. The distributed Bragg reflectors at opposite sides of the active semiconductor layer are formed from alternating high refractive index and low refractive index quarter-wavelength thick semiconductor or dielectric layers that function as mirrors. The multiple layers between the opposing distributed Bragg reflectors, including the active semiconductor layer, form a resonant cavity for light emission or light absorption within the semiconductor structure. The active semiconductor layers within the resonant cavity will either emit light for a light emitting diode (LED) or vertical cavity surface emitting laser (VCSEL) or absorb light for a photodetector (PD).
The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.
A forward biased electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to “inject” them into the active layers, where electrons recombine with holes to produce light.
A light emitting diode (LED) will emit light from the resonant cavity through one of the mirrors through either the upper or lower surface of the semiconductor structure. The mirror reflectivities are typically made lower than lasers to allow efficient light emission.
For a laser (VCSEL), optical feedback provided by the opposing mirrors allows resonance of some of the emitted light within the resonant cavity to produce amplified stimulated emission of coherent “lasing” through one of the mirrors through either the upper or lower surface of the semiconductor structure.
For a photodetector (PD), a reverse biased electrical potential is applied through the electrodes between the p-side and the n-side of the layered structure. A photodetector will absorb light in the active layer of the resonant cavity, thereby generating electron/hole pairs at the active layer. The generated carriers are collected at the device electrode at either the upper or lower surface of the semiconductor structure forming a photocurrent signal.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III–V semiconductors, comprise elements selected from group III, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as aluminum gallium indium nitride (AlGaInN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters or light detectors for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.
These materials are particularly suited for use in short-wavelength VCSELs or LEDs or PDs for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III–V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
Semiconductor resonant cavity structures comprising nitride semiconductor layers grown on a sapphire substrate will emit or absorb light in the near ultra-violet to visible spectrum within a range including 280 nm to 650 nm, allowing better efficiency and narrower line widths for LEDs and photodetectors.
The shorter wavelength blue of nitride based semiconductor VCSELs and LEDs provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) VCSELs and LEDs for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue light emitting devices can potentially be combined with existing red and green lasers or LEDs to create projection displays and color film printers.
In many applications, the conventional substrate material for semiconductor structures would be silicon or gallium arsenide. However, the GaN crystal structure, combined with the high GaN growth temperatures, make deposition of high-quality nitride semiconductor material directly onto semiconductor substrates such as Si or GaAs very difficult.
Nitride based semiconductor structures currently require heteroepitaxial growth of GaN thin layers onto dissimilar substrates such as sapphire or silicon carbide.
A problem specific to fabricating GaN VCSELs and resonant cavity LED's and photodetectors is the difficulty in growing the highly reflective AlGaN-based distributed Bragg reflectors (DBRs) needed for stimulated emission of coherent light of VCSELs or the emission or collection of light for resonant cavity LED's and PD's, where the minimum aluminum content for the AlGaN layers in the DBRs is limited by self absorption of the light and the maximum aluminum content is limited by lattice matching constraints.
Similar problems plague the long wavelength indium phosphide VCSELs but the problem in phosphide based laser structures can be solved by etching a hole through the substrate and evaporating dielectric materials to form the DBR. Unfortunately, the usual substrate for nitride based structures, i.e. sapphire, is difficult to dry or wet etch, so that this back-etch procedure is not available to the fabrication of the nitride based resonant cavity structure.
It is an object of the present invention to provide highly reflective mirrors on both sides of the nitride based resonant cavity semiconductor structure.